1. Field of the Invention
The present invention relates to an electronic system in which a high direct current (dc) voltage of a high voltage battery is converted into a three-phase alternating current (ac) voltage in switching elements under control of a control unit to supply electric power of the ac voltage to a motor for a vehicle.
2. Description of Related Art
An electric power converter having switching elements and a control circuit is disclosed in Published Japanese Patent Specification No 3,430,878. In this Specification, the converter has an insulated-gate bipolar transistor (IGBT), a first metal-oxide-semiconductor field effect transistor (MOSFET), a second MOSFET, a third MOSFET and a control circuit. The first MOSFET has a source connected with a power source of the driving circuit, a drain connected with a gate of the IGBT and a gate connected with the control circuit. Each of the second and third MOSFETs has a source connected with an emitter of the IGBT, a drain connected with a gate of the IGBT and a gate connected with the control circuit.
The control circuit controls the three MOSFETs according to a driving signal sent from an outer part to drive the IGBT. When receiving the driving signal instructing the IGBT to be set in the on state, the control circuit turns on the first MOSFET while turning off the second MOSFET. Therefore, electric charge is supplied from the power source to the gate of the IGBT. As a result, the gate voltage of the IGBT becomes higher than an on-off threshold value, and the IGBT is turned on.
In contrast, when receiving the driving signal instructing the IGBT to be set in the off state, the control circuit turns off the first MOSFET while turning on the second MOSFET. Therefore, the charge accumulated at the gate of the IGBT is released to the ground through the second MOSFET. As a result, the gate voltage of the IGBT becomes lower than the on-off threshold value, and the IGBT is turned off. Further, when the gate voltage becomes equal to or lower than a predetermined value lower than the on-off threshold value, the control circuit turns on the third MOSFET. Therefore, the charge of the gate of the IGBT is further released to the ground through the third MOSFET. As a result, the IGBT keeps being in the off state.
However, even when the first MOSFET set in the on state receives a turning-off instruction from the control circuit so as to be turned off, the first MOSFET sometimes fails in being turned off so as to keep being in the on state. This failure is called an on-failure in this specification. Therefore, even when the second MOSFET is turned on, the failed first MOSFET keeps the gate voltage of the IGBT at a high value. In this case, the IGBT is set in the abnormal state. When the abnormal state of the IGBT is continued, the IGBT continues generating heat, and the IGBT is sometimes damaged or broken by the generated heat.